Almost six hundred million years ago, long before vertebrates existed, some sea-dwelling animals evolved a hard, armor-like body covering, and things just haven't been the same since. Those organisms were the ancestors of insects, spiders, centipedes, millipedes, the extinct trilobites, lobsters, and their relatives  an enormously successful group of organisms known as arthropods. And for better or worse, all arthropods have inherited some basic characteristics of those first ancestors, including their armor-like body covering  the exoskeleton (exo = outside).